What you need is a 4 pole constant torque spindle. These give high torque at low rpm and will cut steel no problem.

We recently started stocking and selling various spindles types from Large ATC spindles down to Constant torque manual tool change spindles. We always test products before committing to stock or use to ensure they are up to the job. So we fit a 4 pole 2.2Kw 24Krpm ER20 water-cooled Constant torque spindle with Ceramic bearings on our own router for testing with a view to offering it as an upgrade from the standard 2.2Kw WC spindles we fit on the machines we build and sell and I must say the power of these spindles has blown me away.

We mostly cut aluminum on this router, which in all honesty is a battered and abused hybrid of a machine that I wouldn't dare show to anyone and this spindle has transformed the machine into a monster that munches aluminum like it's cutting butter.
We have increased the DOC to more than double what the standard 2.2Kw could handle and feed by 50% and getting better tool life because now we can actually cut at recommend tool parameters.

The very first thing we did was cut some mild steel at 5000rpm with an 8mm4F tool @ 2.5DOC which it did without complaint, we always watch the Amps when cutting and they hardly changed and were less than half the rated amps.

The spindles we supply are high quality with GMN Ceramic bearings, balanced rotors using quality wire in the windings with very low runout and quality balanced collet nuts.
Now the downside is they are not cheap at £700 but you get what you pay for, however, these are not the typical spindles you see on eBay or Aliexpress and are made to order. We even have a 40Krpm version.!

Forget the Servo idea unless you have a cartridge-type spindle-like that is fit on milling machines. Just too many obstacles to overcome on a router.